1 fluorite like dispersion, 4 super low dispersion
[Tested on a 42 MP camera] It can deliver sufficient resolution to essentially match the detail captured by Sony’s own 100-400 G Master zoom at a considerably lower price, albeit not quite with the same degree of biting contrast [...] The G Master fights back with an aperture that’s one third of a stop brighter [...], closer focusing especially at 400mm, better stabilisation, more confident focusing [...] Sigma’s 100-400mm deservedly earns a recommendation.
It makes that versatility and performance accessible to the average photographer that doesn’t have deep pockets or who does photography for pleasure rather than profit. Sigma has managed to take what was good about the original 100-400 Contemporary and improve on nearly all its weaknesses, and the end result is a well-rounded third party alternative to the much more expensive G Master lens.
Sony Alpha Blog
[Tested on 61 MP camera] It's very good lens for the price and will ideally complement the Sony FE 24-105 F4 G OSS on a A7III, A9 or A7RIII [...] On the Sony A7RIV it can not compete with the Sony FE 100-400mm in term sharpness and AF speed, but on a lower resolution camera like A7III or A7RIII you won’t see noticeable difference [...] The AF is very good on still subject but on subject in movement you get a higher rate of missed shoots than on the Sony 100-400.